The invention relates in general to regulation of the air-fuel ratio in an internal combustion engine, and in particular to a method of and device for regulating the air-fuel ratio by using a lambda probe (or oxygen probe) which is sensitive to the proportion of oxygen in the combusted fuel mixture. The probe delivers practically a binary signal in the sense that for a rich mixture a "high" lambda signal (corresponding to about one volt), and for a weak mixture a "low" Lambda signal (corresponding to about 50 millivolts) is generated at the output of the probe. The switch-over point of the lambda probe in a first approximation corresponds to a unitary lambda value (lambda=1) at which the air-fuel ratio corresponds exactly to the stochiometric value.
The output lambda signal from the oxygen probe is applied to a regulating device which controls via setting members the air-fuel ratio. If the oxygen or lambda probe detects a rich mixture then the regulating device controls the fuel supply so that after a certain delay which is determined by the propogation time of the air-fuel mixture through the engine, a weaker fuel mixture is indicated by the lambda signal at the output of the probe. As a consequence, the fuel metering system of the engine serves again to produce a richer mixture until the oxygen probe indicates an excessively rich mixture. In this building up condition the output lambda signal of the oxygen probe continuously fluctuates between the "high" and "low" states.
In the known lambda regulators of this kind, for processing this fluctuating output signal of the oxygen probe, a rotary speed adaptive PI regulator has been used among other measures. The values of the P- and I-components of this regulator cannot for various reasons be selected arbitrarily. The reason for this limitation resides in the possibility of an excessive exhaust gas emission due to dynamic mismatch resulting from the above-mentioned transmission or propogation time of the air-fuel mixture through the engine. On the other hand, the run of the engine would exhibit even in a stationary operation only unsatisfactory results.
In order to avoid the above disadvantages and to improve sluggishness of the regulating arrangement, in the German publication DE-OS No. 22 06 276 a lambda regulating device has been disclosed in which the time interval between two change-over processes of the lambda signal at the output of the probe is detected and after the expiration of a preset time interval in which no change-over of the output signal occurs, the device is switched over to another integration time constant, particularly to a lower time constant of the integration regulator.
This known regulating device has proven satisfactory in practice, although an optimum behavior of the power output of the internal combustion engine with respect to the exhaust gas emission has not been achieved. In particular, in different operational situations of the engine it may happen that this known regulating device has an excessively sluggish behavior causing impurity of the exhaust gas and unequality of the driving behavior.